An electrolyte is any substance that produces an electrically conductive solution when dissolved in a solvent, typically water. This property is fundamental to chemistry and the function of living organisms. The ability of a compound to make a solution conductive is the definitive way to identify it as an electrolyte. This concept dictates how substances interact in solution, underpinning processes from industrial applications to biological functions like nerve signaling.
The Chemical Requirement for Conductivity
A solution carries an electric current only when it contains freely moving, electrically charged particles. Unlike metallic wires where current flows via electrons, in a liquid solution, the charge carriers are atoms or molecules with a net positive or negative charge, known as ions. The presence of these ions is the prerequisite for electrical conductivity in water.
When an ionic compound, such as table salt (sodium chloride), dissolves in water, the process is called dissociation. Water molecules overcome the electrostatic attraction holding the ions together in the solid structure, pulling them apart. The separated ions (Na+ and Cl-) are then solvated by water, allowing them to move independently throughout the solution.
For polar covalent compounds, like certain acids, the process is termed ionization. These molecules are not initially composed of ions, but the strong pull from the water solvent causes the covalent bond to break, forming new ions. Whether through dissociation or ionization, the result is the generation of mobile positive and negative ions that migrate toward oppositely charged electrodes when a voltage is applied, allowing electricity to flow.
Classifying Strength: Strong, Weak, and Non-Electrolytes
The amount of current a solution can carry depends directly on the concentration of ions present, leading to a classification system based on the extent of ionization or dissociation. Compounds fall into three categories based on how thoroughly they break apart in water. This classification helps predict a compound’s electrolytic behavior.
A compound is categorized as a strong electrolyte if it dissociates or ionizes nearly 100% when dissolved in water. This complete separation generates a high concentration of ions, allowing the solution to conduct electricity well. Examples include soluble salts like sodium chloride (NaCl), strong acids like hydrochloric acid (HCl), and strong bases like sodium hydroxide (NaOH).
Weak electrolytes only partially ionize or dissociate in solution, often at levels between 1% and 10%. This partial separation means most of the original compound remains as intact, uncharged molecules, resulting in a much lower concentration of free ions. A solution containing a weak electrolyte conducts electricity poorly compared to a strong electrolyte solution. Weak acids, such as acetic acid, and weak bases, like ammonia (NH₃), are typical examples.
Non-electrolytes include compounds that dissolve in water but do not produce any ions at all. These substances remain as neutral, intact molecules when solvated. Since there are no charged particles available to carry an electric current, solutions of non-electrolytes, such as sugar (glucose) and ethanol, do not conduct electricity.
Practical Methods for Testing Conductivity
While chemical knowledge can predict a compound’s classification, the most definitive way to determine if a substance is an electrolyte is through a physical test of the solution’s electrical conductivity. A simple laboratory setup can measure this property empirically.
One common method involves a conductivity apparatus that uses a light bulb connected to a circuit with two electrodes submerged in the solution. If the electrodes are placed in a non-conductive liquid, the circuit remains open, and the bulb does not light up. A strong electrolyte allows for a strong current flow due to high ion concentration, causing the bulb to glow brightly. A weak electrolyte, with limited ion production, allows only a small current to pass, resulting in a dim glow.
A more precise method uses a conductivity meter, which directly measures the solution’s ability to carry a current. The reading is displayed in units such as Siemens per meter (S/m) or milliSiemens per meter (mS/m). The meter applies a voltage between two electrodes and measures the resulting current flow, which is proportional to the ion concentration. A high numerical reading confirms a strong electrolyte, a low but non-zero reading indicates a weak electrolyte, and a reading near zero confirms a non-electrolyte.